The large drop of threshold during the first 250 milliseconds of dark adaptation was suggested by Brown, Watanobe, and Murakami (Cold Spring Harbor Symposium on Quantitative Biology, 30, 1965) to be based on the decay of the receptor potential from the adapting light following extinction of the light. During the current year we investigated whether an intense bleachlight would cause the early cone dark adaptation curve to change its shape by losing its immediate early drop in the same way that the cone receptor potential tends to remain up briefly when an intense adapting light is terminated. We found that it does so. Studies during the 01 year of the grant already have shown that a similar circumstance obtains in the case of the rods--we used a rod monochromat as subject, to avoid intrusion of cone activity at the adapting intensities necessary to saturate the rods. The Brown idea seems to provide a good understanding of the physiological basis of early dark adaptation. We now intend to make direct comparisons between threshold changes and cone pigment concentration changes, measured by retinal densitometry, during light adaptation. Our aim is to see if early dark adaptation (which we now presume reflects receptor potential changes) does or does not show an increased extent when the photopigment bleaching during light adaptation has not reached equilibrium.